This invention relates to a method and a system for treatment of biochemical waste, and more particularly to an improvement of a method of anaerobic treatment of the biochemical waste.
Recently, in accordance with the spread of secondary treating facilities for sewage and waste-water sludge from stock breeding industry, a large amount of surplus activated sewage is continuously produced. In addition, garbage from the human life accumulates to a large amount.
The biochemical waste is mainly formed of the activated sewage and the garbage. Raw sewage contains organic material formed of fats, proteins, carbohydrates and microorganisms. Accordingly, it is very important to efficiently treat the biochemical waste which essentially consists of animal and plant cells and microorganisms.
The anaerobic digestion process has been popular in the waste treatment. It is characterized by the capability of treating large volumes of dilute organic slurries at low cost, because the produced gas can be used for motive power of treating facilities, the high kill rate of pathogenic organisms and the capability of producing solid residues suitable for use as soil conditioners.
Despite its many advantages, the anaerobic digestion process has not yet reached its full potential, because the anaerobic digestion process normally requires long terms such as 40 days for a low treatment efficiency. Therefore nowadays the anaerobic digestion process is not used vigorously.
If the rise of the treatment efficiency can be attained, the anaerobic digestion process is very useful for its many advantages. Various high speed digestion processes which can treat the biochemical waste in fourteen or fifteen days have been provided. But those processes unavoidably sacrifice their digestion rates. This means that those processes have advantages referring to a volume decrease of waste and a rise of dehydration, but they do not satisfy the requirements referring to the useful utilization of digestion gas and the level of BOD (biochemical oxygen demand) of the separated liquid.
The surplus activated sewage is treated by the anaerobic digestion process at a temperature of from 20.degree. to 50 C. for long periods of time while the sewage is concentrated to about from 1 to 570 solids by a settling basin or a centrifuge. During the stay, sewage bacteria are gradually liquefied by the action of anaerobic bacteria to convert to methane gas through organic acid. The methane-producing bacteria are among strictly anaerobic organisms known.
It is well know among those people in this art that if during the stay the sewage in a state of slurry is not well stirred, the digestion remarkably delays and the removing rate of BOD becomes low.
General biochemical waste including surplus activated sewage is a kind of hydrophilic colloid and in a state of almost hydrated gel. Therefore, the sewage of solid density of 5% presents a state of paste and needs a large stirring power. Accordingly, actually the sewage of solid density up to about 5% only have been treated.
The need for improving anaerobic digestion technology is therefore greater now than ever before.